In the drilling of a borehole through the earth's crusts to penetrate oil or gas bearing formations several types of drill bits are utilized. One category of drill bits is that of the rolling cone or rolling cutter drill bit. Such a bit usually utilizes three of such rolling cone cutters rotatably mounted on downward extending journals each of which protrudes from one of three legs extending downwardly at the lower end of the bit body. In the rolling cone bit category there are basically two types of cutter constructions. The first type is the "milled tooth" bit wherein the conical cutters have protruding cutting elements or milled teeth formed on the surface thereof from the same basic piece of blank stock as the cone. The second category of rolling cone drill bits involves the "insert" type of bits wherein the cones are made of one material and have drilled recesses in the surfaces for receiving hard metal cutting elements termed inserts.
Each type of rolling cone drill bit has advantages and disadvantages. The milled tooth type of bit is advantageous in that broad flat sharpened tooth shapes can be formed on the cutters to provide a wide sharply penetrating cutting action on the bottom hole. These broad flat sharp milled teeth are also tough and fracture resistant since they are made out of the same tough alloy as the cone and are integral parts thereof. The disadvantage in the milled tooth cutter type of bit is that the teeth are particularly susceptible to wear from abrasion and erosion of the alloy in the extended tooth area.
The second type of bit, the insert bit, offers the advantage of the hard metal cutting elements or inserts which are tremendously resistant to such abrasive forces. Usually the inserts are made of a very hard material such as tungsten carbide sintered and compacted into a generally cylindrical-frusto conical shape. Holes are usually bored into the conical cutter to receive the cylindrical end of the insert and the generally frusto conical portion of the insert protrudes from the cutter surface. The disadvantage of the insert type bits is that the inserts generally are not as fracture resistant as the milled tooth cutting elements and therefore cannot be shaped as broad and flat and sharp as the milled teeth. Thus the bottom hole coverage and penetration rate of the insert is less desirable than that of the milled tooth although the insert generally will wear many times longer than the milled tooth.
The conventional insert bits manufactured today generally utilize three rolling cones having circumferential rows of inserts securely attached to the cones by interference fit within the holes bored substantially perpendicular to the surface of the cone. These conventional cutter cones have rows of inserts in circumferential rows around the conical surfaces of the cones. One of the problems incurred in this conventional insert pattern is that because of the deeply bored insert recesses in the conical surfaces a weakening of the cone structure is effected. This weakening must be offset by a thickening of the cone resulting in a circumferential land passing around the cone in the area of the insert locations. This adds to the weight and reduces the effective size of the allowable bearing surface on which the cone is mounted. In addition to the problem of the weakening of the cone structure, which weakening is particularly susceptible to hoop stresses in the cone structure, the insert type construction also suffers from an effect known as gyration when the bit is used to drill relatively tough formations.
Gyration occurs because of the circumferential rows of inserts forming grooves in the rock face being drilled. These parallel grooves leaves one or more raised annular ridges of rock material called a kerf. When this kerf becomes high enough it causes these rows of inserts to follow the grooves formerly cut by the other cutter inserts and results in the drill bit following a non-central axis of rotation. This results in an orbital action of the bit termed "gyration" and is a destructive force on the drill bit. Likewise, the gyration effect reduces the cutting speed of the bit to a negligible amount. The kerf buildup eventually contacts the non-cutting surfaces of the cones and totally stops any cutting action of the bit in the hole. Likewise, the gyration forces introduced are not those for which the bit is designed and as a result, unusual damage usually occurs to the inserts, the cones and the bearings. The present invention overcomes these disadvantages by providing a drill bit cone structure having a unique insert pattern which reduces failures from hoop stresses on the cone structure and greatly prevents gyration and tracking of the conical cutters of the drill bit. This pattern of insert placement on the conical cutters is a series of non-linear circumferential or annular bands of inserts on the cutter surfaces.